Various medical procedures require the accurate localization of a three-dimensional position of a surgical instrument within the body in order to effect optimized treatment. For example, some surgical procedures to fuse vertebrae require that a surgeon drill multiple holes into the bone structure at specific locations. To achieve high levels of mechanical integrity in the fusing system, and to balance the forces created in the bone structure, it is necessary that the holes are drilled at the correct location. Vertebrae, like most bone structures, have complex shapes including non-planar curved surfaces making accurate and perpendicular drilling difficult. Conventionally, a surgeon manually holds and positions a drill guide tube by using a guidance system to overlay the drill tube's position onto a three dimensional image of the bone structure. This manual process is both tedious and time consuming. The success of the surgery is largely dependent upon the dexterity of the surgeon who performs it.
When a surgeon performs minimally invasive surgery and prepares to place a screw into bone, it is often desirable to insert a dilator tube from the surface of the skin through muscle and connective tissue down to a position where the distal end of the dilator tube is adjacent to bone. The dilator tube serves as a corridor through which drilling and other surgical steps can occur. Currently, the most frequently used method for gauging whether the dilator is sufficiently far enough inserted is to record x-ray images, which is effective but exposes the patient and surgical staff to x-rays and can be time consuming. If image guidance were to be used instead of x-rays, it would spare the patient and staff some exposure to x-rays. However, a tracking array mounted on a dilator tube has the drawbacks that it is unwieldy and obtrusive to the surgeon in an area where a great deal of surgical activity is occurring.
Thus, there is a need to be able to measure the depth of surgical instrumentation in a manner that limits the exposure of the patient and medical staff to unnecessary radiation from imaging systems without obstructing and impeding a medical staff's ability to perform the surgical operation on the patient. The present disclosure overcomes the disadvantages of current traditional surgical techniques and robot-assisted surgical techniques. For example, using known positions of components of a robot surgical system allows a surgeon to position a long shaft such as a dilator tube within a secondary tracked tube, such as the robot's end effector guide tube or a free tracked external guide tube, to a depth such that the dilator tube contacts the target bony structure. This positioning would allow tracking the dilator tube indirectly without tracking the dilator itself and without the use of x-ray images as it is further inserted into a patient.